EVOLUTION OF THE TROPOSPHERIC SPLIT JET OVER THE SOUTH-PACIFIC OCEAN DURING THE 1986-89 ENSO CYCLE

A case study investigation into the meridional and horizontal circulat ion over the South Pacific Ocean is presented for the 1986-89 El Nino- Southern Oscillation (ENSO) cycle. Using the European Centre for Mediu m-Range Weather Forecasts (ECMWF) analyses, annual average fields are created for the years before and after the 1987 minimum (warm phase) a nd 1989 maximum (cold phase) in the Southern Oscillation index. The an alyses reveal a shift in the split jet stream over the South Pacific s ector (180 degrees-120 degrees W) from a strong subtropical jet can) a nd weak polar front jet (PFJ) during the warm phase to a weak STJ and strong PFJ during the cold phase. Analysis of the momentum budget reve als how the split jet in the upper troposphere over South Pacific Ocea n evolved during the 1986-89 ENSO cycle. During the warm phase: the st rong STJ is associated with advection of the mean flow momentum Aux fr om the Australian sector, which is approximately balanced by a large n egative ageostrophic term; the PFJ is primarily associated with eddy m omentum convergence, which is partially counterbalanced by the ageostr ophic term. During the cold phase, the weakened STJ is related to an i ncreasingly negative ageostrophic term and a less positive mean flow m omentum convergence. The strengthened PFJ is associated with an increa se in the convergence of eddy momentum flux that is mainly composed of 2.5-6-day poleward momentum transport from midlatitudes and 7-30-day equatorward momentum transport from high latitudes. In general, the im pacts of eddy stress on the STJ and the mean momentum divergence on th e PFJ in this sector are small. The variations in the split jet may re flect the poleward propagation of the ENSO signal via the South Pacifi c convergence zone. The implications for the high southern latitudes a re discussed as interannual variations are found in the low-level east erlies near Antarctica and the Amundsen Sea low.